In carrying out a piping construction for a gas supply, a water supply or an air-conditioner at a wall face, a floor face, a foundation or the like of a building formed by concrete, the wall face or the like is bored by using a concrete drill driven by power of electric power, compressed air or the like. By applying rotation and vibration from a side of the concrete drill to a core bit mounted to a chuck portion formed at a front end of the concrete drill, a hole having a predetermined diameter is bored at a concrete wall or the like by a boring blade in a cylindrical shape formed at a front end of the core bit. Various core bits are prepared in accordance with a diameter and a depth of a hole to be bored, and used by being mounted to the chuck portion of the concrete drill in accordance with the diameter of the hole and the thickness of the wall to be bored.
As described in, for example, JP-A-11-309710 (page 3, FIG. 1 and FIG. 2), a core bit of a background art is provided with a cutter ring at a front end of a drill portion having a ring-like supporting member and formed with a portion thereof for being mounted to a concrete drill on an opposed end side, and the mounting portion is formed with a guide portion in a cylindrical shape and a locking portion projected from an outer peripheral face of the guide portion to an outer side in a radius direction. The core bit is mounted to a chuck portion on a side of a tool by the mounting portion to transmit rotation and vibration in an axial direction from the side of the tool to a side of the core bit via the locking portion.
However, according to the above-described core bit of the background art, the locking portions mounted to the chuck portion of the concrete drill for transmitting the rotational force from the concrete drill to the side of the core bit are formed to project to the outer side in the radius direction at equal intervals in a peripheral direction and therefore, there is a case of damaging an outer shape of the locking portion when the core bit is erroneously dropped, or by being brought into contact with other object by vibration or the like in containing or carrying the core bit. When the outer shape of the locking portion is deformed by damage, there is a case in which the core bit cannot be mounted to the chuck portion of the concrete drill or mounted thereto in an incomplete state and the core bit is disengaged from a mounted state in boring operation to be accompanied by a danger. Further, there is a case in which when mounting is incomplete, seal of a cooling water for cooling the cutter ring portion becomes incomplete to leak water from a bonding face of the concrete drill and the core bit.